The present invention relates to a fuel injector used in the delivery of fuel to a cylinder of a diesel internal combustion engine of the type in which fuel is supplied to a high pressure accumulator (the “common rail”) by a suitable pump and is delivered from the accumulator to the fuel injectors of the engine, the nozzles of which are arranged to actuate in turn to deliver fuel to the respective cylinders of the engine.
Such fuel injectors generally comprise a needle which is slidable within a body and engageable with a needle valve seat to control the flow of fuel from a high pressure fuel supply line through the body.
The maximum injection pressures within a fuel injector may be of the order of 1800 bar or higher and as a consequence the forces to be overcome in order to lift the needle of the injector are large. It is not therefore possible to directly control the injector using an electromagnetic actuator unless very high currents are used. The injector is therefore indirectly controlled by means of a valve arrangement which controls the pressurising or discharging of a control chamber located above the injector needle.
An example of such an injector is disclosed in EP0647780 in which the end of the needle remote from the valve seat extends within a chamber, the chamber being arranged to receive fuel from the supply line through a restrictor. In use, injection is controlled by varying the pressure within the control chamber. A solenoid actuator acts upon a valve arrangement to cause a flow path between the control chamber and a low pressure drain to open. As the pressure falls within the control chamber the needle leaves the needle valve seat due to pressure acting against a portion of the needle adjacent the valve seat.
Within common rail injection systems two types of valve arrangements are known, pressure-balanced valve arrangements (sometimes referred to as equilibrium valves) and non-pressure balanced valve arrangements.
In a pressure balanced valve arrangement a valve stem located within a bore is slidable under the action of the electromagnetic actuator to open and close a flow path between the high pressure region of the control chamber and the low pressure region of a low pressure drain. In the closed configuration the valve arrangement is in contact with a valve seat and is substantially in hydraulic equilibrium, with the valve arrangement being held in the closed position by the action of a spring on the valve arrangement. Upon actuating the electromagnetic actuator the spring force is overcome and the valve arrangement moves away from its seat thereby allowing fuel to move between the stem and bore to the low pressure drain. Equilibrium based valve arrangements tend to demonstrate a degree of static leakage. In other words, even in the closed position high pressure fuel will leak along the flow path defined between the bore of the valve arrangement and the valve stem to the low pressure drain.
In a non-pressure balanced valve arrangement the valve is held in its seated and closed position by the pressure of the high pressure fuel within the system. Such a valve arrangement is not therefore substantially in hydraulic equilibrium in the closed position and consequently requires a greater activation force in order to open. However, the degree of static leakage within such a non-pressure balanced valve arrangement is lower than in the pressure balanced valve arrangement.
The two types of valve arrangement described above therefore either have a low actuation force requirement with a relatively high degree of static leakage (pressure balanced valve arrangement) or high actuation force requirement with a relatively low degree of static leakage (non-pressure balanced valve arrangement).
It is an object of the present invention to provide an improved injector arrangement that has a low actuation force requirement but which has improved static leakage performance by acting directly on the needle.